For information on the human brain specifically, please see its article.

In animals, the brain, or encephalon (Greek for "in the head"), acts as the control center of the central nervous system. In most animals, the brain is located in the head close to the primary sensory apparatus and the mouth. While all vertebrate nervous systems have a brain, invertebrate nervous systems may have either a centralized brain or collections of individual ganglia. The brain is an extremely complex organ; for example, the human brain is a collection of 100 billion neurons, each linked with up to 25,000 others [1]. This huge number of interconnecting neurons—often referred to as a neural ensemble—is what allows the brain to conduct such complex processes.

Most brains exhibit a visible distinction between grey matter and white matter. Grey matter consists of the cell bodies of the neurons, while the white matter consists of the fibers (axons) that connect neurons over long distances. The entire outer visible layers of the brain is called the cortex which consists primarily of grey matter. However, deeper grey matter structures called nuclei also exist throughout the central nervous system. The axons of this white matter are surrounded by a fatty insulating sheath called myelin, giving the white matter its distinctive color.

The study of the brain is known as neuroscience, a field of biology aimed at understanding the functions of the brain at every level, from the molecular up to psychological.

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A distinction is sometimes made in the philosophy of mind between the mind and brain. The brain is defined as the physical, biological matter contained within the head, responsible for all electrochemical neuronal processes. The mind, however, exists as something outside of the brain. The mind is sometimes thought of as consciousness, the soul, or some other non-physical center of thought.

Early views on the function of the brain regarded it as little more than cranial stuffing. In Ancient Egypt, from the late Middle Kingdom onwards, in preparation for mummification, the brain was regularly removed, for it was the heart that was assumed to be the seat of intelligence. According to Herodotus, during the first step of mummification, "The most perfect practice is to extract as much of the brain as possible with an iron hook, and what the hook cannot reach is mixed with drugs." Over the next five-thousand years, this view came to be reversed; the brain is now known to be seat of intelligence, although idiomatic variations of the former remain, as in memorizing something "by heart".[1]

The first thoughts of the field of psychology actually came from ancient philosophers, including Aristotle. As philosophers became more in tune with medical research over time, the idea of psychology emerged. From that point, different branches of psychology emerged with different individuals creating new ideas.

Modern neuroscience is experiencing rapid growth due to the availability of computers capable of handling the intense processing required for understanding such a complex system. Neuroscientists use a variety of approaches to study the brain at different levels—from the molecules to systems. Extensive knowledge has been accumulated about the electrophysiological properties of different types of neurons and their responsiveness to neurotransmitters. Recordings from the brains of awake, behaving animals pioneered by Edward Evarts help to decode neuronal firing during different behaviors and cognitive processes. Miguel Nicolelis introduced multielectrode recording techniques which led to creation of rudimentary brain-computer interfaces. Rapidly developing neuroimaging techniques such as allows scientists to study the brain in living humans and animals in ways that their predecessors could not.

The structure of the human brain is different from that of other animals in several significant ways. These differences have allowed for many abilities over and above those of other animals, such as advanced cognitive skills. Human encephalization is especially pronounced in the neocortex, the most complex part of the cerebral cortex. The proportion of the human brain that is devoted to the neocortex—especially to the prefrontal cortex—is larger than in all other animals.

Humans enjoy unique neural capacities, but much of the human brain structure is shared with ancient species. Basic systems that alert the nervous system to stimulus, that sense events in the environment, and monitor the condition of the body are similar to those of the most basic vertebrates. The neural circuitry underlying human consciousness includes both the advanced neocortex and prototypical structures of the brain stem. The human brain also has a massive number of synaptic connections allowing for a great deal of parallel processing.

Despite the variance of the species in which the brain is found there are many common features in its cellular make-up, its structure, and its function. On a cellular level the brain is composed of two classes of cells, neurons and glia, both of which contain several different cell types which perform different functions. Interconnected neurons form neural networks (or neural ensembles). These networks are similar to man-made electrical circuits in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). These do not form simple one-to-one electrical circuits like many man-made circuits, however. Typically neurons connect to at least a thousand other neurons [2]. These highly specialized circuits make up systems which are the basis of perception, action, and higher cognitive function.

Neurons are the cells that generate action potentials and convey information to other cells; these constitute the essential class of brain cells. In each particular brain area, input (or afferent) neurons, output (or efferent) neurons, and interneurons are typically found. Input neurons are recipients of projections from other brain areas. Output neurons project to the other areas. Interneurons are the neurons which do not leave the area but rather perform local processing.

In addition to neurons, the brain contains glial cells in a roughly 10:1 proportion to neurons. Glial cells (Greek: “glue”) perform supportive roles to neurons including creating the insulating myelin, providing structure to the neuronal network, waste management, and neurotransmitter clean up. Most types of glia in the brain (and the rest of the central nervous system) are present in the entire nervous system. Exceptions include oligodendrocytes which insulate neural axons (a role performed by Schwann cells in the peripheral nervous system). Oligosaccharides are the defining factor between white matter and grey matter in the brain—white matter is composed of myelinated axons, whereas grey matter contains mostly cell soma, dendrites, and unmyelinated portions of axons and glia. The space between neurons is filled with dendrites as well as unmyelinated segments of axons; this area is referred to as the neuropil.

In mammals, the brain also contains connective tissue called the meninges, a system of membranes that separate the skull from the brain. This three-layered covering is made of, from the outside in, dura mater, arachnoid mater, and pia mater. The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid, a substance that protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. A blood-brain barrier protects the brain from toxins that might enter through the blood.

The brain is suspended in cerebrospinal fluid (CSF), which circulates between layers of the meninges and through cavities in the brain called ventricles. It is important both chemically for metabolism and mechanically for shock-prevention. For example, the human brain weighs about 1-1.5 kilograms. The mass and density of the brain are such that it will begin to collapse under its own weight. The CSF allows the brain to float, easing the stress caused by the brain’s mass.

Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are then interpreted throughout the central nervous system reactions are formulated based upon reflex and learned experiences. A similarly extensive nerve network delivers signals from a brain to control muscles throughout the body. Anatomically, the majority of afferent and efferent nerves (with the exception of the cranial nerves) are connected to the spinal cord, which then transfers the signals to and from the brain.

To control movement the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by the motor cortex, cerebellum, and the basal ganglia. The system eventually projects to the spinal cord and then out to the muscle effectors. Nuclei in the brain stem control many involuntary muscle functions such as heart rate and breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone.

Brains also produce a portion of the body's hormones that can influence organs and glands elsewhere in a body—conversely, brains also react to hormones produced elsewhere in the body. In mammals, most of these hormones are released into the circulatory system by a structure called the pituitary gland.

It is hypothesized that developed brains derive consciousness from the complex interactions between the numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on midbrain and limbic functions as well. Among "younger" (in an evolutionary sense) vertebrates, advanced processing involves progressively rostral (forward) regions of the brain.

Hormones, incoming sensory information, and cognitive processing performed by the brain determine the brain state. Stimulus from any source can trigger a general arousal process that focuses cortical operations to processing of the new information. This focusing of cognition is known as attention. Cognitive priorities are constantly shifted by a variety of factors such as hunger, fatigue, belief, unfamiliar information, or threat. The simplest dichotomy related to the processing of threats is the fight-or-flight response mediated by the amygdala and other limbic structures.

Clinically, death is defined as an absence of brain activity as measured by EEG. Injuries to the brain tend to affect large areas of the organ, sometimes causing major deficits in intelligence, memory, and movement. Head trauma caused, for example, by vehicle and industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is caused by resultant swelling (edema) than by the impact itself. Stroke, caused by the blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage.

Several areas of science specifically study the brain. Neuroscience seeks to understand the nervous system, including the brain, from a biological and computational perspective. Psychology seeks to understand behavior and the brain. The terms neurology and psychiatry usually refer to medical applications of neuroscience and psychology respectively. Cognitive science seeks to unify neuroscience and psychology with other fields that concern themselves with the brain, such as computer science (artificial intelligence and similar fields) and philosophy.

Each method for observing activity in the brain has its advantages and drawbacks. Electrophysiology, in which wire electrodes are implanted in the brain, allows scientists to record the electrical activity of individual neurons or fields of neurons. However this method requires invasive surgery and thus this technique is typically usually used only with lab animals or during neurosurgery.

By placing electrodes on the scalp one can record the summed electrical activity of the cortex in a technique known as EEG. EEG measures the mass changes in electrical current from the cerebral cortex, but can only detect changes over large areas of the brain with very little sub-cortical activity.

Functional magnetic resonance imaging (fMRI) measures changes in blood flow in the brain, but the activity of neurons is not directly measured, nor can it be distinguished whether this activity is inhibitory or excitatory. Similarly, a positron emission tomography (PET), is able to monitor glucose metabolism in different areas within the brain which can be correlated to the level of activity in that region.

Behavioral tests can measure symptoms of disease and mental performance, but can only provide indirect measurements of brain function and may not be practical in all animals. In humans however, a neurological exam can be done to determine the location of any trauma, lesion, or tumor within the brain, brain stem, or spinal cord.

Autopsy analysis of the brain allows for the study of anatomy and protein expression patterns, but is only possible after the human or animal is dead. Magnetic resonance imaging (MRI) can be used to study the anatomy of a living creature and is widely used in both research and medicine.

Attempts have also been made to directly "read" the brain, which has been accomplished in a rudimentary manner through a brain-computer interface. Brain activity can be detected by implanted electrodes, raising the possibility of direct mind-computer interface. The reverse method has been successfully demonstrated: brain implants have been used to generate artificial hearing and (crude and experimental) artificial vision for deaf and blind people. Brain pacemakers are now commonly used to regulate brain activity in conditions such as Parkinson's disease.

Computer scientists have produced simulated neural networks loosely based on the structure of neuron connections in the brain. Artificial intelligence seeks to replicate brain function—although not necessarily brain mechanisms—but as yet has been met with limited success.

Creating algorithms to mimic a biological brain is extremely difficult because the brain is not a static arrangement of circuits, but rather a network of vastly interconnected neurons that are constantly changing their connectivity and sensitivity. More recent work in both neuroscience and artificial intelligence models the brain using the mathematical tools of chaos theory and dynamical systems. Current research has also focused on recreating the neural structure of the brain with the aim of producing human-like cognition and artificial intelligence.

The neurons of the brain require a lot of energy. Although the brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization. The energy consumption for the brain to simply survive is 0.1 calories per minute, while this value can be as high as 1.5 calories per minute during crossword puzzle-solving.[3] The demands of the brain limit its size in many species. Molossid bats and the Vespertilionid Nyctalus spp. have brains that have been reduced from the ancestral form to invest in wing-size for the sake of manoeuverability. This contrasts with fruit bats, which require more advanced neural structures and do not pursue their prey.[4]

↑Hendrickson, Robert (April 2000). The Facts on File Encyclopedia of Word and Phrase Origins, New York: Facts on File. "The ancient Greeks believed that the heart, the most noticeable internal organ, was the seat of intelligence and memory as well as emotion. This belief was passed on down the ages and became the basis for the English expression 'learn by heart,' which is used by Chaucer (1374) and must have been proverbial long before that. 'To record' reminds us again of this ancient belief in the heart as the seat of the mind. When writing wasn't a simple act, things had to be memorized; thus we have the word 'record,' formed from the Latin 're,' 'again,' and 'cor,' 'heart,' which means exactly the same as 'learn by heart.'"